Simplified offset current reducting circuit for auto kine bias (akb)

ABSTRACT

The disclosed embodiments relate to a current reduction circuit that reduces offset current from at least one driver circuit. The current reduction circuit comprises circuitry that measures the offset current and produces a measurement current and circuitry that receives the measurement current and responds by maintaining an auto kine bias measurement voltage within a predetermined range.

FIELD OF THE INVENTION

This invention relates to the field of auto kine bias (AKB), and inparticular, to an offset current reducing circuit for AKB

BACKGROUND OF THE INVENTION

This section is intended to introduce the reader to various aspects ofart which may be related to various aspects of the present inventionwhich are described and/or claimed below. This discussion is believed tobe helpful in providing the reader with background information tofacilitate a better understanding of the various aspects of the presentinvention. Accordingly, it should be understood that these statementsare to be read in this light, and not as admissions of prior art.

A cathode ray tube (CRT) driver integrated circuit, such as the PhilipsTDA6120 CRT driver integrated circuit, can be used as a CRT driver. TheTDA6120 has a cathode current sample output pin that can be used forAKB. The design of the measurement circuit is such that there can be asmuch as +/−30 microamperes of offset current added to the desired CRTcathode measurement current for each of the red, green and blue drives.However, a back end signal and sync processor such as the ToshibaTA1316AN has an AKB sampling system with a limited dynamic range, andcannot handle the potential total of +/−90 microamperes of offset (3X+/−30 μA). The AKB will not function under this condition. Thisinvention is intended to remove enough of the offset current that theremainder is within the dynamic range of the measurement circuit of theTA1316AN. Data sheets of detailed operation for the TDA6120 and theTA1316AN are available from Philips and Toshiba respectively.

The invention represents an improvement over a concept used previouslyutilized in the manufacture of televisions. That concept, when modifiedfor projection TV, required three current clamps, one on each of thered, green and blue CRT drive boards. While this arrangement works verywell, it is complicated and has a large number of parts. Moreover, ifany one of the three clamps malfunctions the AKB system will not operatecorrectly. This can cause the instrument to shut down. Accordingly,there is a need to simplify this design to provide the same performance,yet at the same time, to be more robust and be subject to fewerproblems. Moreover, reducing the number of parts can significantlyreduce the cost of production.

SUMMARY OF THE INVENTION

The disclosed embodiments relate to a current reduction circuit thatreduces offset current from at least one driver circuit. The currentreduction circuit comprises circuitry that measures the offset currentand produces a measurement current and circuitry that receives themeasurement current and responds by maintaining an auto kine biasmeasurement voltage within a predetermined range.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a circuit schematic for an offset current reducing circuit forAKB in accordance with the inventive arrangements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

One or more specific embodiments of the present invention will bedescribed below. In an effort to provide a concise description of theseembodiments, not all features of an actual implementation are describedin the specification. It should be appreciated that in the developmentof any such actual implementation, as in any engineering or designproject, numerous implementation-specific decisions may be made toachieve the developers' specific goals, such as compliance withsystem-related and business-related constraints, which may vary from oneimplementation to another. Moreover, it should be appreciated that sucha development effort might be complex and time consuming, but wouldnevertheless be a routine undertaking of design, fabrication, andmanufacture for those of ordinary skill having the benefit of thisdisclosure.

In accordance with the inventive arrangements, a single comparator canbe utilized instead of the three current clamps used by the previoussystem. More particularly, and in the presently preferred embodiment,the comparator is responsive to a voltage derived from the sampled beamcurrent and a reference voltage. The comparator output is a currentsubstantially free of the influences of offset current, and therefore,assured of remaining within a predetermined dynamic range. The inventivearrangements reduce the offset current from the CRT driver ICsufficiently to allow a back end signal and sync processor's AKBmeasurement circuit, even one subject to a limited dynamic range, tooperate correctly. The inventive arrangements substantially remove bothpositive and negative offset currents. As compared to the previouslydescribed current clamp system, the total part count is advantageouslyreduced by approximately 85 components. Other advantages of theinventive arrangements may be applicable to other kinds of circuits.

With reference to FIG. 1, an offset current reducing circuit 10 isillustrated. The kine current measurement outputs from pin 7 of each ofthree TDA6120 CRT driver ICs 12, 14 and 16 (one each for red, green andblue) are coupled to the node labeled “I_(K) FROM KINE SOCKETS”. Each ofthe kine current measurement outputs from the CRT drivers 12, 14 and 16has a series resistor (20, 22 and 24, respectively) coupled thereto toprovide arc protection. An 11 K ohm resistor 26 is also coupled to thenode and is returned to a +12 volt power supply. The AKB circuit iscompleted by the red, green and blue, low-level drive signals that areoutput from integrated circuit 18 pins 43,42 and 41, respectively, beingapplied to the input pin 2 of respective CRT driver integrated circuits12, 14 and 16. The basic operation of the AKB circuit is for the DC biasof low-level red, green and blue drive signals to be controlled by thevoltage applied to pin 45 of integrated circuit 18 during each channel'smeasurement interval, thus controlling the bias applied to the red,green and blue cathodes 82, 78 and 74, respectively. The cathode currentmeasurement signal at pin 7 of each CRT driver is reflective of thecut-off bias of each cathode connected to pin 12 of that CRT driver. Theerror correction signal for each channel of the AKB control loop is heldon capacitors 80, 76 and 72, which are connected to the red, green andblue filter pins 48, 47 and 46 of integrated circuit 18. The timeconstant associated with each channel's dominant pole is determined bythe value of capacitors 80, 76 and 72 and is typically on the order ofseveral fields, the specific value being chosen for loop stability andnoise performance. The dominant pole of the offset current reductioncircuit is formed by the values of capacitor 56 and resistor 54. Thecharge rate of capacitor 56 is determined by resistor 54 and the valueof transistor 50 collector current, while the discharge rate isdetermined by resistor 54. The time constant of the dominant pole of theoffset current reduction circuit must be much longer than the dominantpole time constant of the AKB loop. In the preferred embodiment theoffset current reduction circuit time constant is several seconds andthe power-on charge is set to stabilize the loop in about four secondsin order to avoid a visible brightness “bounce” when power is applied.Those of ordinary skill in the art will appreciate that the values ofcomponents, component identification numbers, voltage levels and thelike referred to herein and in FIG. 1 are exemplary only. Other valuesmay be used in some cases as a matter of design choice.

A 5V peak-to-peak pulse signal (0V to 5V) whose width is approximately64 microseconds and whose repetition rate is approximately 16.7milliseconds is coupled to a resistor 70. This pulse is an attenuatedversion of the vertical drive pulse. The pulse occurs shortly after thestart of the vertical blanking interval. At this time, the total CRTbeam current is theoretically zero. As a practical matter, there is asmall amount of flyback signal pickup, but it is small enough to beneglected. The pulse saturates a transistor 68, which develops a voltageof nominally 5.88 volts at the base of a transistor 46. This turns on atransistor 46, whose collector current is approximately 900microamperes. The transistor 68 is connected to a voltage source asillustrated in FIG. 1 through a resistor 64 and a resistor 66. Thecollector current of the transistor 46 activates a differentialamplifier 47 made up of a transistor 50 and a transistor 48. The base oftransistor 50 is biased by the divider made up of resistors 58 and 60.This voltage divider establishes a base voltage of approximately 2.1volts. The base of the transistor 50 is additionally coupled to groundvia a capacitor 62. The collector of the transistor 50 is connected toground via a resistor 52 and a resistor 54. The base of transistor 48 isconnected to resistors 34 and 32, capacitor 36, and pin 45 of a ToshibaTA1316AN integrated circuit 18. Pin 45 of integrated circuit 18 is theAKB measurement point. The differential pair measures the differencebetween the divider voltage at the base of transistor 50 and the voltageresulting from the sum of the CRT cathode currents, the offset currentsfrom the CRT driver ICs, the current provided by the 11 K ohm resistor26 and the collector current of transistor 40. The offset currentcomponent must be substantially removed for AKB to function properly.

If the “zero current” voltage at pin 45 of the TA1316AN 18 at the timeof the vertical rate pulse can be held between −0.5 volts and 3 volts,an internal clamp in the TA1316AN 18 can substantially reduce theremainder of the offset. However, the total voltage offset couldtheoretically be between −2.43 volts (−90 microamperes×27 K ohms) and+2.43 V (+90 microamperes×27 K ohms). One could, for example, simplypump approximately 70 microamperes of current into the node, and thatwould raise the minimum to −0.5 volts. However, the maximum would thenbecome 4.36 volts, and this value is outside the dynamic range of themeasurement system. If, however, the voltage at pin 45 can be maintainedat approximately 2 volts, the system will function properly.

If the voltage at the base of transistor 48 is lower than the voltage atthe base of transistor 50, indicating a negative offset current, thecollector current of transistor 50 will be reduced, and the voltage oncapacitor 56 and the base of transistor 40 will fall. This will reducethe collector current of transistor 40. The current of transistor 40 issubtracted from the sum of the “zero beam” current during the verticalpulse measurement period, the offset current, and the current that flowsthrough the 11 K ohm resistor 26. Because the current in transistor 40is reduced, the current through resistors 32 and 34 increases, and thevoltage at pin 45 of the TA1316AN 18, which was low initially,increases.

Conversely, if the voltage at the base of transistor 48 is higher thanthe voltage at the base of transistor 50, indicating a positive offsetcurrent, the current in transistor 50 will increase, which will raisethe voltage on the base of transistor 40. This increases the current intransistor 40, which reduces the current through resistors 32 and 34,dropping the voltage at pin 45 of the TA1316AN 18. The 11 K ohm resistor26 advantageously adds an offset in the total current so that transistor40 is always active over the total range of offset currents from the CRTdriver ICs.

In an AKB system it is desirable to limit the voltage at the measurementpoint that results from high cathode currents that can occur duringactive video, since the cathode current always flows in the system,including negative first derivative currents resulting from dischargingthe parasitic cathode capacitances. This limiting of the measurementpoint voltage is advantageously accomplished in this system by passingthe current through resistor 32 as well as resistor 34. As the sum ofthe cathode currents increases, the drop across these two resistors willeventually saturate transistor 30, the collector of which is connectedto the node labeled “I_(K) FROM KINE SOCKETS” by a resistor 28. Thevoltage drop across the resistors 32 and 34 limits the voltage on pin 45of the TA1316AN 18 to approximately 4 volts, and eliminates the need fora Zener diode or PNP emitter follower to clip the measurement voltageduring active scan. A capacitor 36 is connected from integrated circuit18 pin 45 to ground to filter any residual video rate signals at pin 45.

Development of the circuit resulted in several advantageous circuitcomponent configurations and value selections that were not apparent inthe initial design stage. The original value of resistor 44 was 5.6 Kohms. This value was reduced to 2.7 K ohms to improve the transientresponse of the clamp. Resistor 32 was originally 1 K ohm, but thisvalue made the saturated collector voltage of transistor 30 too high andsomewhat unpredictable. A value of 39 K ohm for resistor 32 provides alower, more stable saturated collector voltage.

The Zener diode 38 was not originally in the system, but was added whenit became apparent that the negative currents associated with thecrosshatch pattern (discharging the parasitic cathode capacitances)caused the voltage at pin 45 of the TA1316AN 18 to try to go belowground. However, the base to collector junction of transistor 40 becameforward biased before the voltage could get to ground via a resistor 42.This would start discharging capacitor 56 and cause the displayedpicture to go green because of dynamic range limitation of the AKBmeasurement system. During active scan with a 100 IRE crosshatchpattern, the lower voltage at the base of transistor 40 would decreaseits collector current, which resulted in higher current in resistor 34,thus raising the voltage at pin 45 of the TA1316AN 18. The clamp was notfast enough to restore the charge on the capacitor, and the voltageremained below the normal equilibrium point. With this condition, thegreen AKB pulse would start to go beyond the linear range of the AKBdetector, and the system would increase the green video's bias level tocompensate. This caused the green shift observed. With a Zener diode 38installed, the collector of transistor 40 is advantageously always at orabove 2.2V. This is sufficiently high to keep the base collectorjunction from becoming forward biased. It also advantageously allows theintegrated circuit 18 pin 45 voltage to operate at the desired 2 voltsDC since the level at pin 45 of the TA1316AN resulting from a voltage atthe collector of transistor 40 of 2.2 volts is 0.9V. This means that theZener is advantageously off when the AKB current measurement is made.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and will be described in detail herein. However,it should be understood that the invention is not intended to be limitedto the particular forms disclosed. Rather, the invention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the invention as defined by the following appended claims.

1. A current reduction circuit that reduces offset current from at leastone driver circuit, the current reduction circuit comprising: circuitrythat measures the offset current and produces a measurement current; andcircuitry that receives the measurement current and responds bymaintaining an auto kine bias measurement voltage within a predeterminedrange.
 2. The current reduction circuit set forth in claim 1, whereinthe at least one driver circuit comprises three cathode ray tube (CRT)driver circuits, each of the CRT driver circuits corresponding to acolor beam in a CRT display device.
 3. The current reduction circuit setforth in claim 2, wherein each of the CRT driver circuits comprises aPhilips TDA6120 CRT drive integrated circuit.
 4. The current reductioncircuit set forth in claim 1, wherein the auto kine bias measurementvoltage is provided to a Toshiba TA1316AN integrated circuit.
 5. Thecurrent reduction circuit set forth in claim 4, wherein the auto kinebias measurement voltage corresponds to an offset current that is withinthe dynamic range that may be processed by the Toshiba TA1316ANintegrated circuit.
 6. The current reduction circuit set forth in claim1, wherein the predetermined range is between about −0.5 volts and 3.0volts.
 7. The current reduction circuit set forth in claim 1, whereinthe circuitry that measures the offset current comprises a circuit thatgenerates a reference voltage that is compared to a signal correspondingto the offset current to generate the measurement current.
 8. Thecurrent reduction circuit set forth in claim 1, wherein the currentreduction circuit comprises a portion of a television.
 9. The currentreduction circuit set forth in claim 1, wherein the offset current has amaximum range of about +/−90 microamperes.
 10. A cathode ray tube (CRT)display device that is adapted to display images, comprising: a CRT;three driver circuits, each of the driver circuits being associated witha color beam adapted to create an image on the CRT, each of the threedriver circuits producing a signal indicative of an offset current; acircuit that generates a reference voltage; a differential amplifierthat receives the reference voltage and the signal indicative of theoffset current from the three driver circuits and responds by producinga differential output that is proportional to the difference between thereference voltage and the signal corresponding to the offset currentfrom the three driver circuits; and circuitry that receives thedifferential output and responds by maintaining an auto kine biasmeasurement voltage within a predetermined range.
 11. The CRT displaydevice set forth in claim 10, wherein each of the driver circuitscomprises a Philips TDA6120 CRT drive integrated circuit.
 12. The CRTdisplay device set forth in claim 10, wherein the auto kine biasmeasurement voltage is provided to a Toshiba TA1316AN integratedcircuit.
 13. The CRT display device set forth in claim 12, wherein theauto kine bias measurement voltage corresponds to an offset current thatis within the dynamic range that may be processed by the ToshibaTA1316AN integrated circuit.
 14. The CRT display device set forth inclaim 10, wherein the predetermined range is between about −0.5 voltsand 3.0 volts.
 15. The CRT display device set forth in claim 10, whereinthe CRT display device comprises a portion of a television.
 16. The CRTdisplay device set forth in claim 10, wherein the offset current fromthe three driver circuits has a total maximum range of about +/−90microamperes.
 17. A method of reducing offset current from at least onedriver circuit, the method comprising: measuring an offset currentassociated with the at least one driver circuit; producing a measurementcurrent in response to the measurement of the offset current; employingthe measurement current to maintain an auto kine bias measurementvoltage within a predetermined range.
 18. The method of reducing offsetcurrent set forth in claim 17, comprising generating a referencevoltage.
 19. The method of reducing offset current set forth in claim18, comprising comparing the reference voltage to a signal proportionalto the offset current.
 20. The method of reducing offset current setforth in claim 17, wherein the recited acts are performed in the recitedorder.